1. Field of the Invention
The present invention relates to a photochemical vapor phase reaction apparatus and a method of causing a photochemical vapor phase reaction, more particularly relates to an apparatus for carrying out chemical vapor deposition and etching assisted by optical energy.
2. Description of the Prior Art
Among techniques for depositing thin films in vapor phase, photo-CVD has been well known as an excellent method which can deposit excellent quality thin films at low temperatures with little damage to the underlying surface as compared with other available CVD techniques such as thermal CVD, plasma CVD.
An exemplary conventional apparatus is shown in FIG. 1. The apparatus comprises a vacuum chamber defining a reaction (deposition) space 2 therein, an evacuating system 8 including a rotary pump 19, a reactive gas feeding system 7, a mercury bubbler 13 for activating reactive gases supplied from the gas feeding system 7, a substrate holder 1' for supporting substrates 1 thereupon, a heating means 3 for maintaining the substrate temperature at an appropriate level, and a low pressure mercury lamp 9 for irradiating the substrates through a window 10 made of a quartz plate. In the operation of the apparatus, a reactive gas such as disilane is introduced into the reaction chamber 2 after evacuating the deposition space 2 in the chamber to an appropriate pressure by means of the evacuating system 8. The reactive gas is decomposed by the optical energy of ultraviolet light emitted from the lamp 9 and leaves a thin film such as an amorphous film on the substrates 1 which are heated to 250.degree. C. by the heating means 3.
The deposition of the product of the reaction, however, takes place also on the surface of the light window 10 so that the transparency thereof is gradually reduced as the deposition of the thin film on the substrates 1 continues. The deposited product on the window comes to absorb substantial part of the ultraviolet light and decelerates the deposition speed on the substrates 1 and eventually deposition is stopped. Accordingly, there is a limit to the thickness of films which can be formed by this method. One of the resolution of this problem is a coating 16 of Fomblin oil (fluorine-based oil) on the window for retarding the progress of the unwanted deposition. The oil coating tends, on the other hand, to get mixed within the films deposited on the substrates 1 and degrade the quality of the films deposited. Furthermore, the oil coating can not completely prevent the unwanted deposition so that a little product is deposited at a slow deposition speed, posing a similar problem of the limit to the maximum thickness of the films on the substrates 1.